Many electronic products, such as laptop, desktop, PDA and so on, need direct current (DC) power supply. With the development of electronic technology, fast load transient response is needed. But some problem may be introduced with fast load transient response, e.g., overshoot of an output voltage.
As shown in FIG. 1, take a constant-ON time controlled step-down converter as one example, at time T0, a load current IO increases at a fast rate, then an output voltage VO decreases quickly, an inductor current IL increases slowly, and at least one switch is periodically turned ON at a constant-ON time period until output voltage VO increases to an output target DVO. For simplicity, inductor current IL shown in FIG. 1 represents an average inductance current. Due to an output capacitor, output voltage VO is not in-phase but has a phase delay to inductor current IL. When output voltage VO increases to output target DVO at time T1, inductor current IL becomes much higher than load current IO and would charge output voltage VO to a higher value, as a result, an overshoot of output voltage VO happens.